The present invention relates to a cathode ray tube having an electron gun employing an indirectly heated cathode, and in particular to a highly-reliable long-life cathode ray tube having prevented occurrence of leakage current by improving insulating characteristics between a cathode sleeve and a heater of the indirectly heated cathode.
Cathode ray tubes used for a color television receiver, a display monitor and the like are widely used in various fields as display means because of their capability of reproducing high-definition images.
Cathode ray tube of this kind includes a vacuum envelope formed of a panel portion, a neck portion and a funnel portion for connecting the panel portion and the neck portion, a phosphor screen formed of phosphors coated on an inner surface of the panel portion, an electron gun housed in the neck portion, and comprised of a plurality of electrodes such as an indirectly heated cathode, a control electrode and an accelerating electrode for projecting an electron beam toward the phosphor screen, and a deflection yoke mounted around the funnel portion for scanning the electron beam emitted from the electron gun over the phosphor screen. The electron gun usually employs an indirectly heated cathode.
FIG. 5 is a cross-sectional view of an essential part of an indirectly heated cathode and its vicinity of a prior art cathode ray tube. In FIG. 5, reference numeral 51 denotes an indirectly heated cathode structure, the indirectly heated cathode structure 51 comprises a tubular cathode sleeve 52, a cap-shaped cathode cap 53 fixed at an end of the cathode sleeve 52, an electron-emissive material layer 54 coated on a top surface of the cathode cap 53, and a heater 55 a portion of which is disposed within the cathode sleeve 52 for heating the cathode cap 53.
A portion of a spirally wound heating wire 55a of the heater 55 is covered with an insulating film 55b made chiefly of alumina and a coating film 55c containing alumina and tungsten powder. Of the insulating film 55b and the coating film 55c, the insulating film 55b covers all the heating wire 55a of the heater 55 extending to ends 55e except for end portions 55d for welding, and the coating film 55c covers the outer surface of approximately all the insulating film 55b except for the vicinity of the ends 55e of the insulating film 55b extending from a coil portion 55f on the side of the top of the cathode sleeve 52 to ends 55g beyond a flared bottom end 52a of the cathode sleeve 52.
The coating film 55c contains a small amount of tungsten powder as described above and appears black, and the insulating film 55b is made chiefly of alumina and appears white, but the heater 55 appears black as a whole, and this type of heaters are generally called dark heaters.
The heater 55 is welded to heater supports 56 at its end portions 55d for welding. The cathode sleeve 52 is fixed to a small-diameter portion of a cathode cylinder 58, a large-diameter portion of which is fixed to a tubular cathode support eyelet 57. The cathode support eyelet 57 and the heater supports 56 are fixed to a pair of multiform glasses 61 via bead supports 59 and via heater lead straps 60, respectively. Reference numeral 62 denotes a control electrode which is fixed to the multiform glasses 61 with a desired spacing between it and the electron-emissive material layer 54.
The techniques for employing such dark heaters are described in the following references, for example.
Japanese Patent Publication No. Hei 8-3976 (published on Jan. 17, 1996) discloses a technique for improving withstand voltage characteristics by preventing deformation and cracking of an insulating alumina film of a heater using insulating alumina powder of specified average diameters.
Japanese Patent Application Laid-open No. Hei 7-161282 (laid-open on Jun. 23, 1995) discloses a technique for suppressing a leakage current between a heater and a cathode by combining a dark heater with a cathode sleeve having a silicon carbide film on its inner surface.
Japanese Patent Application Laid-open No. Hei 11-213859 (laid-open on Aug. 6, 1999) discloses a technique for suppressing a leakage current between a heater and a cathode by dispersing at least one of niobium and tantalum in a film made of a mixture of tungsten and alumina and coated on at least one of an inner surface of a cathode sleeve and a surface of the heater.
Japanese Patent Application Laid-open No. Hei 11-273549 (laid-open on Oct. 8, 1999) discloses a technique for suppressing a leakage current between a heater and a cathode by improving purity of alumina used for insulation of the heater and thereby increasing electrical resistance of the alumina itself.
Japanese Utility Model Publication No. Sho 60-3483 (Jan. 31, 1985) discloses a technique for preventing cracking of alumina by extending a dark-film region to cover a three-layer winding portion of each leg portion of a heater.
Cathode ray tubes employing such dark heaters have a feature in that heat can be efficiently radiated from a heater because the outer surface of the heater is darkened and thereby heat radiation efficiency of the surface of the heater is increased, and consequently, their reliability can be improved.
However, the prior art structure shown in FIG. 5, or the techniques disclosed in the above-cited references are not sufficient for preventing the leakage current between the heater and the cathode. In an automatic cutoff-voltage control circuit for controlling a cathode current to a predetermined value and used in a color television receiver or a display monitor, the leakage current between the heater and the cathode is superposed on the cathode current. Consequently, there is a problem in that, if the predetermined value of the cathode current in the color television receiver or the display monitor is not sufficiently large compared with a value of the leakage current between the heater and the cathode, the automatic cutoff-voltage control circuit cannot control the cutoff voltages of the electron beams for three colors of red, green and blue, a balance among the three colors is lost such that white balance is not obtained, the automatic cutoff-voltage control circuit is inoperable and the adjustment of the receiver or the monitor becomes difficult.
If the leakage current between the heater and the cathode begins to flow, the alumina film serving as a heater insulating film is heated by the leakage current, oxygen escapes from the alumina due to the heat, and electrical conductivity occurs in the oxygen-deficient alumina (Al2O2.99). As a result, there are various problems, and the heater is sometimes broken by a further increase in the leakage current, and therefore it is important in view of ensuring reliability of a cathode ray tube to prevent the leakage current between the heater and the cathode.
The following two causes are confirmed for occurrence of the leakage current between the heater and the cathode.
As for a first one of the two causes, it was found out that, in cathode ray tubes rejected for the leakage current between the heater and the cathode, many insulating films 55b which should otherwise be white have turned gray. The analysis confirmed that the cause of this coloration is tungsten.
Tungsten present within a cathode ray tube is used in the heating wire 55a of the heater 55 and the above-mentioned coating film 55c. If the two are compared with each other, tungsten contained in the coating film 55c is of a small powder size of about 1.0 xcexcm in diameter, and is chemically active compared with the heating wire 55a. 
The degree of vacuum of the cathode ray tube is poorest immediately after flashing of getters in the manufacturing step, that is, about 10xe2x88x922 Pa. After flashing of the getters, decomposition of residual gases within the tube by an electron beam and adsorption of the residual gases by the getter film provide the ultimate degree of vacuum of about 10xe2x88x925 Pa. It was found out that the mean free paths of the residual gases are about several tens cm in the poorest degree of vacuum (about 10xe2x88x922 Pa) and the residual gases react with portions having directly exposed tungsten within the tube.
It was confirmed from the above facts that the residual gases collide with fine tungsten powder especially in the portions of the dark coating film 55c extending from the vicinities of the flared bottom end 52a of the cathode sleeve 52 outwardly to the ends 55g of the dark coating film 55c, then the tungsten is dispersed into the alumina of the insulating film 55b, the alumina is brought into a semiconductor state by the water cycle phenomenon (see Horikoshi, G.: xe2x80x9cVacuum Technology (second edition),xe2x80x9d chap. 4.2.8, p. 85, Tokyo University Press, for example.), and thereby the alumina film produces an electrical conductivity and increases the leakage current between the heater and the cathode.
A second one of the two causes for occurrence of the leakage current between the heater and the cathode is occurrence of the leakage current due to the physical contact between the heater and the cathode sleeve. This is caused by the fact that leg portions of the heater 55 are pulled apart when the leg portions of the heater 55 are welded to heater supports 56 and the contact area between the heater and the cathode sleeve 52 is increased in the vicinity of the flared bottom end 52a of the cathode sleeve 52.
It is an object of the present invention to provide a superior cathode ray tube having prevented the leakage current between the heater and the cathode by solving the above problems with the prior art.
To achieve the above object, the present invention provides a superior cathode ray tube having prevented the leakage current between the heater and the cathode by specifying a relationship between a cathode sleeve of an indirectly heated cathode of an electron gun and a coating length of a coating film of the heater inserted in the cathode sleeve such that collisions and consequent reactions between the residual gases within the tube and the coating film of the heater are reduced and at the same time the contact area between the heater and the cathode sleeve is reduced.
In accordance with an embodiment of the present invention, there is provided a cathode ray tube having an evacuated envelope including a panel portion, a neck portion, a funnel portion for connecting the panel portion and the neck portion and a stem having a plurality of pins therethrough and being sealed to close the neck portion at one end thereof, a phosphor screen formed on an inner surface of the panel portion, an electron gun housed in the neck portion, the electron gun having an indirectly heated cathode structure and a plurality of electrodes disposed downstream of the indirectly heated cathode structure, spaced specified distances apart, arranged axially in a specified order, and fixed by insulating rods for projecting an electron beam toward the phosphor screen, and a deflection yoke mounted around a vicinity of a transitional region between the neck portion and the funnel portion for scanning the electron beam on the phosphor screen, the indirectly heated cathode structure comprising: a base metal having an electron emissive material coating on an outer top surface thereof; a metal sleeve having the base metal attached to a first end of the metal sleeve and having a second end opposite from the first end; a heater housed partly within the metal sleeve, the heater including a major heating portion having a spirally wound heating wire and leg portions connected to respective ends of the major heating portion and comprising heating wires wound spirally in a plurality of layers; an insulating film covering the major heating portion and a portion of each of the leg portions continuous with the major heating portion; and a black coating film covering a portion of the insulating film extending from the major heating portion toward each of the leg portions, a whole of the black coating film being housed within the metal sleeve.